The use of metal thin films and filaments is very common in measurement and sensing devices, such as flow and velocity sensors. In such measuring and sensing devices, often a hot conductor thin filament that can dissipate heat is held in a fluid stream. When the conductor filament loses heat to the fluid stream, its temperature falls. The resulting change in electrical resistance of the conductor filament associated with the fall in temperature can be measured and it would be indicative of the flow characteristics of the fluid stream. Often such conductor filaments are made from conductor thin films. For making the conductor filament (or thin film), a variety of metals can be used. For such measurement and sensing applications, platinum is preferred over other metallic substances because of its refractory nature as well as its high electrical resistance and temperature coefficient of resistance (TCR). It is also inert and can operate at a high temperature.
In some applications, because of the catalytic nature of platinum, to prevent unwanted chemical reaction, it is desirable to completely encapsulate the platinum in a thin dielectric film such as silicon nitride or silicon oxynitride. The dielectric also serves as a support material for the thin film platinum. However, because of the inert nature of platinum, it does not adhere well to most dielectric materials. Furthermore, the platinum thin film is made by deposition on a surface and during the subsequent fabrication of the measurement or sensing device, it may be necessary to anneal the deposited thin film at a high temperature. Annealing improves the material properties of thin film platinum. It typically reduces the resistance and increases the thermal coefficient of resistance of the thin film to close to the bulk value. The annealing typically takes place at temperatures at or above 800.degree. C. At this high temperature, due to thermal expansion mismatch of the thin metal film with the substrate (i.e., the material supporting the thin metal film), high stress results in the film and at the interface. The stress can be so high as to cause the metal to totally come off (delaminate) or blister from the substrate. If a platinum thin film is used to form one of the layers of electrical contact pads, the poor adhesion of the platinum on the underlying dielectric material causes a weak link that can result in bond pad failure. On a metal film attached to a dielectric material having a different coefficient of expansion from that of the metal film, blistering and protrusions (hillocks) can form when the metal film experiences high temperature fluctuation. What is needed a technique for forming a secure adhesion of the conductor metal (especially platinum) on the dielectric material, and methods to reduce the interfacial stress of the conductor metal on the dielectric material so that delamination and blistering are prevented or greatly reduced.
Various groups have conducted investigation on improvement of adhesion of platinum to silicon nitride layers and reduction of stress by texturing the surface before platinum deposition. Documents of interest related to metal film adhesion to dielectric material in sensors include U.S. Pat. No. 4,501,144 (Higashi et al.), U.S. Pat. No. 4,683,159 (Bohrer et al.), U.S. Pat. No. 4,696,188 (Higashi), U.S. Pat. No. 4,891,977 (Johnson et al.), and U.S. Pat. No. 4,952,904 (Johnson et al.). Documents of interest related to hillock formation include U.S. Pat. No. 3,986,897 (McMillan et al.) and U.S. Pat. No. 4,012,756 (Chaudhari et al.).